Dr.Noura Abouammoh
Dr. Afnan Younis
Define and calculate measure of frequency of disease (Prevalence, Incidence, Attack rate)
Be able to decide when to use them Be able to interpret the results
5 Ws.
So what? - Prevention and control
Distinguish between case and an episode- Case e.g. Poliomyelitis.- Episode e.g. an asthma attack requiring admission.
If 75 cases of TB in village A and 25 cases in village B, is TB commoner in village A?
The total number of cases of a disease in a given population at a specific time.
Prevalence = No. of cases with the disease at a point in time/Total no. of people in defined pop. at same point in time X 10n
Point vs. period
1000 men were working in factory A were screened for HIV on 1 January 2002 and 50 of them were found to be positive for HIV antibodies. The screening was repeated in the same 1000 men on 1 January 2003 and this time 62 men were positive, including the 50 men who were positive on the first screening.
What is the prevalence of HIV in men working in factory A on 1 January 2002 and 1 Jan 2003?
No. of prevalent cases at 1 January 2002= 50
No. persons in the pop. In January 2002=1000
Therefore, the prevalence at January 2002=50 / 1000 X 1000 = 50/1000
No. of prevalent cases at January 2003= 62No. of persons in the population at 1
January 2003= 1000
Therefore, the prevalence at 1 January 2003= 62/1000 X 1000 = 62/1000
Parents of 700 children in village A were asked if their children have developed diarrhoea during the past 3 weeks. 200 children had a positive history of diarrhoea.
Calculate the prevalence.
No. of cases = 200 No. of population = 700 Prevalence = 200/700 X 1000 =
285/1000
Frequency of new cases of disease in a defined pop. during a specified time period
Incidence= No. of new cases with disease in a specified time period/ No. of disease free people at the start of the time period X 10n
Disease free people = pop. at risk
Prevalence:- Useful in health care settings e.g.
estimate services required.
- Cannot study cases that got better or died.
Incidence:- Useful for investigating causal
relationships and risk factors.
1000 men were working in factory A were screened for HIV on 1 January 2002 and 50 of them were found to be positive for HIV antibodies. The screening was repeated in the same 1000 men on1 January 2003 and this time 62 men were positive, including the 50 men who were positive on the first screening.
What is the incidence of HIV infection in men working in factory A in 2003?
No. of new cases in 2003= 12No. of persons at risk of HIV infection in
January 2003 = 1000 – 50 = 950
The incidence of developing HIV infection = 12/950 X 1000= 12.6/1000
An attack rate is a variant of an incidence
rate, applied to a narrowly defined population
observed for a limited time, such as during an
epidemic.
The attack rate is usually expressed as %
percent.
Of 100 persons who attended a dinner party, 35 subsequently developed gastroenteritis.
Calculate the attack rate of
gastroenteritis
Attendees = 100ILL = 35Attack rate = (35 ÷ 100) X 100 = 35%
In a university student has got H1N1 flu. She attended a class of 80 students, of which 12 students got the flu 5 days later.
Calculate the attack rate of the flu.
Class = 80ILL :• new cases= 12• old case = 1• At risk students= 80 - 1 = 79Attack rate = (12 ÷ 79) X 100 = 15%
Top Related